Measurement of the Mass of the Higgs Boson

Let’s use the word “Higgs boson” for the new state discovered by ATLAS and CMS. The collider physics community is trying to measure everything they can about this new particle. One of the “easiest” properties to measure is its mass, MH. (One of the more difficult is the CP Nature of the Higgs boson.)

For theorists, the exact value (125? 126?) is generally of little concern, because the real mystery is why MH is not closer to the Planck scale. For good reason they have thought hard about this, inventing several possible explanations why the Higgs mass is near the electroweak scale and not at the Planck scale.

For experimentalists, on the other hand, the data are there to make a precise measurement, so we should do so.

As the reader surely knows by now, signals for the “Higgs boson” have been established in several channels, including di-photons, a pair of Z bosons decaying to four charged leptons (total), a pair of W bosons decaying to two charged leptons and two neutrinos, and possibly some evidence in the b-bbar mode. Of these modes, only the di-photon and the four-lepton modes provide a narrow peak at the putative Higgs boson mass, so the CMS and ATLAS collaborations derive their mass measurements from those channels only.

When the Higgs decays to two photons, two energetic and narrow showers are recorded in the electromagnetic calorimeter (“EM calorimeter”). The EM calorimeters of CMS and ATLAS are especially advanced in design and capability — years ago physicists had the decay mode H→γγ in mind when they designed these two wonderful devices. They are truly state-of-the art.

When the Higgs decays to four leptons, we mean electrons and/or muons, not tau leptons, because the Z boson masses and kinematics can be fully reconstructed for the Z→ee and Z→μμ decay modes. So the four leptons can be four electrons, four muons, or two electrons and two muons. The electron energies are measured primarily by the EM calorimeters, so those measurements are quite good. The muon momenta are measured mainly by the precision Silicon detectors which provide several precise points along the trajectory of the muon through a well-known and very uniform magnetic field. Once again, the ATLAS and CMS spectrometers are exceptionally performant, thanks both to the precision of the points (a few microns each) and the high magnetic fields (2 – 4 Tesla). The decay mode H→4L was a leading consideration in the design of these two spectrometers. (CMS and ATLAS also have very advanced dedicated muon detectors mounted outside the calorimetry, that play the essential role in distinguishing muons from other charged particles. For very high momenta, the muon detectors also contribute to the muon momentum measurement, but for the momenta occurring in H→4L decays, they do not contribute much.) For CMS, the measurement resolution (the estimated error for a single lepton) is about the same for electrons and muons produced in Higgs decays. (At higher momenta, though, electrons are more precise than muons, while at lower energies, muons are more precise. This is why we reconstruct J/ψ decays with muons, while heavy Z’ boson decays will show up as narrower peak in the ee channel than in the μμ channel.)

It turns out that the statistical precision on the mass measurement for H→γγ and for H→4L is about the same. This did not have to be the case – the statistical precision depends on the measurements errors, on the size of the signal and on the size and shape of the background. Here is the CMS plot comparing the H→γγ and for H→4L mass measurements.

CMS measurements of the Higgs boson

The green curve shows the H→γγ measurement, and the red curve, H→4L. They plainly are compatible, justifying their combination by CMS; this combination is represented by the black curve.

(You can find this and other plots at the CMS public web site for the combination of CMS Higgs searches.)

There is a subtle point about the combination: should the two channels be given a weight corresponding to the rate expected in the standard model, or should the weight depend on the observed rates? The H→γγ rate is a bit higher than expected, and the H→4L rate is very slightly lower. (It used to be significantly lower, but the signal strength is now quite close to unity.) CMS have produced a two-dimensional plot that allows the normalization of the modes to be free independent of the masses:

2D scan in the plane of signal strength and mass

Within the present uncertainties, it does not make much difference. (The black curve in the first figure above was computed taking the ratio of rates for H→γγ and H→4L from the standard model, but allowing the normalization of the two together to float.)

The statistical uncertainty is at the level of 0.3% (δM/M), so one has to be careful about systematic uncertainties. Is the absolute photon and electron scale, determined by the EM calorimetry, and the absolute muon scale, determined by the magnetic field, accurate at the level of a fraction of a percent?

Needless to say, much effort was expended in calibrating these absolute energy/momentum scales. The copious production of Z bosons, decaying the electron and muon pairs, provides the key to getting the scale right: the Z mass is known at the 10-4 level, thanks to resonant depolarization techniques applied at LEP. Narrow J/ψ and Υ resonances provide cross checks of the momentum scale. Uniformity of response is achieved with huge sets of isolated and well-measured tracks. There is a real art to this, and decades of experience. One should keep in mind the essential role played by the people who design, care for, and calibrate the detectors, without which no clever analysis would produce results this good.

The details of the way systematic uncertainties are handled when combining H→γγ and H→4L are not yet public. One should take into account the degree of correlation between the two mass peaks coming from possible errors on the EM calorimeter energy scale. This correlation will not be large, however.

CMS derived their total systematic uncertainty by eliminating it and seeing how the total uncertainty shrinks. This corresponds to a narrowing of the black parabola in the first plot. (Technically, the systematic uncertainty is eliminated by fixing the nuisance parameters to their best-fit values, and then scanning in MH.) The difference in quadrature of the full width of the parabola and the narrower width obtained when the systematic uncertainty is eliminated is taken as the systematic uncertainty; this is valid so long as there is no correlation between the statistical and systematic uncertainties, and as long as the minimum of the parabola stays basically in the same place. Here is a comparison of the parabola with and without the systematic uncertainty:

parabolas with and without the systematic uncertainty

In this manner the CMS Collaboration arrived at their updated and preliminary measurement of the Higgs mass, MH = 125.8 ± 0.4(stat) ± 0.4(sys) GeV. Taken in its entirety, this measurement is precise at the level of 0.5%. One can expect this value to be refined somewhat as the measurement is improved by including more data and perhaps reducing systematic uncertainties.